The invention relates to methods and apparatus for modifying ice adhesion strength between ice and selected materials. More particularly, the invention relates to systems and methods which apply electrical energy to the interface between ice and such materials so as to either increase or decrease the ice adhesion strength to facilitate desired results.
Ice adhesion to certain surfaces causes many problems. For example, excessive ice accumulation on aircraft wings endangers the plane and its passengers. Ice on ship hulls creates navigational difficulties, the expenditure of additional power to navigate through water and ice, and certain unsafe conditions. The need to scrape ice that forms on automobile windshields is regarded by most adults as a bothersome and recurring chore; and any residual ice risks driver visibility and safety.
Icing and ice adhesion also causes problems with helicopter blades, and with public roads. Billions of dollars are spent on ice and snow removal and control. Ice also adheres to metals, plastics, glasses and ceramics, creating other day-to-day difficulties.
Icing on power lines is also problematic. Icing adds weight to the power lines which causes power outages, costing billions of dollars in direct and indirect costs.
In the prior art, methods for dealing with ice adhesion vary, though most techniques involve some form of scraping, melting or breaking. For example, the aircraft industry utilizes a de-icing solution such as Ethyl Glycol to douse aircraft wings so as to melt the ice thereon. This process is both costly and environmentally hazardous; however, the risk to passenger safety warrants its use. Other aircraft utilize a rubber tube aligned along the front of the aircraft wing, whereby the tube is periodically inflated to break any ice disposed thereon. Still other aircraft redirect jet engine heat onto the wing so as to melt the ice.
These prior art methods have limitations and difficulties. First, prop-propelled aircraft do not have jet engines. Secondly, rubber tubing on the front of aircraft wings is not aerodynamically efficient. Third, de-icing costs are extremely high, at $2500-$3500 per application; and it can be applied up to about ten times per day on some aircraft!
The above-referenced problems generally derive from the propensity of ice to stick and form onto surfaces. However, ice also creates difficulties in that it has an extremely low coefficient of friction. Each year, for example, ice on the roadway causes numerous automobile accidents, costing both human life and extensive property damage. If automobile tires gripped ice more efficiently, there would likely be fewer accidents.
It is, accordingly, an object of the invention to provide systems and methods which modify ice adhesion strength beneficially.
A further object of the invention is to provide systems for reducing ice adhesion on vehicle surfaces such as aircraft wings, ship hulls and windshields to facilitate ice removal.
Still another object of the invention is to provide systems for increasing the coefficient of friction between ice-clad roads and automobile tires, and between ice and other objects such as shoe soles and cross-country skis.
These and other objects will become apparent in the description which follows.
Certain of above-referenced problems would be lessened if the ice adhesion strength were decreased between the ice and the surface upon which the ice forms. For example, if the adhesion strength between the ice and an aircraft wing were decreased sufficiently, wind pressure, buffeting or light manual brushing would remove the ice from the wing. Similarly, scraping an automobile windshield so as to be free of ice would be much less difficult if the ice adhesion strength between the ice and the windshield were lessened.
Other above-referenced problems would be lessened if the ice adhesion strength between ice and surfaces in contact with the ice were increased. For example, if the ice adhesion strength were increased between automobile tires and icy roadways, then there would be less slippage and fewer accidents.
Ice has certain physical properties which allow the present invention to selectively modify the adhesion of ice to conductive (and semi-conductive) surfaces. First, ice is a protonic semiconductor, a small class of semiconductors whose charge carriers are protons rather than electrons. This phenomenon results from hydrogen bonding within the ice. Hydrogen bonding occurs because the hydrogen atoms of water molecules in ice share their electrons with an oxygen atom. Thus, the nucleus of the water moleculexe2x80x94uniquely a single protonxe2x80x94remains available to bond with adjacent water molecules.
Similar to typical electron-based semiconductors, ice is electrically conductive. While this electrical conductivity is generally weak, the conductivity can be altered by adding chemical agents that donate or accept extra charge-carrying particles, i.e., protons in the case of ice.
Another physical property of ice is its evaporability. Evaporability of a substance is a function of vapor pressure at the substance surface. In most materials, vapor pressure drops rapidly at the liquid-to-solid interface. In ice, however, there is virtually no change in vapor pressure at the liquid-to-solid interface. The reason for this is that the surface of ice is covered with a liquid-like layer (xe2x80x9cLLLxe2x80x9d).
The LLL has important physical characteristics. First, the LLL is only nanometers thick. Second, it ranges in viscosity from almost water-like, at temperatures at or near to freezing, to very viscous at lower temperatures. Further, the LLL exists at temperatures as low as xe2x88x92100xc2x0 C., and thus practically exists for most temperatures around the planet.
The LLL is also a major factor of ice adhesion strength. For example, if one brings the smooth surface of ice in contact with the smooth surface of an airplane wing, the actual area of contact between the two surfaces is on the order of one-thousandth of the total interface area between the two surfaces. The LLL functions as a wetting substance between the surfacesxe2x80x94the principal behind almost all adhesivesxe2x80x94and substantially increases the effective contact area between the surfaces. This increase in contact area strongly affects ice adhesion.
The combination of the semiconductive properties of ice and the LLL allows one to selectively manipulate ice adhesion strength between ice and other surfaces. Generally, water molecules within a piece of ice are randomly oriented. On the surface, however, the molecules are substantially oriented in the same direction, either outward or inward. As a result, all their protons, and hence the positive charges, either face outward or inward.
While the exact mechanism is unknown, it is likely that the randomness of water molecules transitions to an ordered orientation within the LLL. However, the practical result of the ordering is that a high density of electrical charges, either positive or negative, occurs at the surface. Accordingly, if a charge is generated on the surface coming on contact with ice, it is possible to selectively modify the adhesion between the two surfaces. As like charges repel and opposites attract, an externally applied electrical bias at the interface of the ice and the other surface either reduces or enhances the adhesion between the ice and the surface.
In one aspect, the invention provides a power source connected to apply a DC voltage across the interface between ice and the surface upon which the ice forms. By way of example, the conductive surface can be an aircraft wing or a ship""s hull (or even the paint applied to the structure). A first electrode connects with the surface; a nonconductive or electrically insulating material is applied as a grid over the surface; and a second electrode is formed by applying a conductive material, for example conductive paint, over the insulating material, but without contacting the surface. The surface area of the second electrode should be small as compared to the overall surface area protected by the system. By way of example, the surface area under protection (i.e., that area sought to be xe2x80x9cice-freexe2x80x9d) should be at least about ten times larger than the surface area of the second electrode.
One or more wires connect the second electrode to the power source; while one or more wires connect the first electrode to the power source. Ice forming over the surface and the conductive grid second electrode completes the circuit. A voltage is then applied to the circuit, selectively, which controllably modifies the ice adhesion strength of the ice with the surface.
A voltage regulator subsystem also preferably connects with the circuit so as to adjustably control the voltage applied across the interface and so as to achieve control over the ice adhesion strength. By way of example, ice made from different concentrations of ions can modify the optimum voltage for which the ice adhesion strength is at a minimum; and the voltage regulator subsystem thereby provides a mechanism by which the minimum can be changed selectively.
Other subsystems preferably connect with the circuit to provide other features, for example to detect whether water or ice completes the circuit. In one aspect, the power source is a DC supply (e.g., a battery) which provides voltage to the circuit and which connects to the deicing electrodes. In another aspect, a DC ammeter connects with the circuit to measure the DC conductivity of the ice (i.e., the semi-conductive layer which xe2x80x9cshortsxe2x80x9d the two electrodes when formed over the surface and any part of the grid second electrode). In another aspect, an AC supply connects with the circuit to generate AC voltages between about 10 kHz and 100 kHz, selectively. According to another aspect, an AC ammeter also connects with the circuit to measure the AC conductivity of the ice at frequencies within the 10-100 kHz range. In still another aspect, a current comparator compares the AC and DC conductivities.
These aspects thus provide circuitry which can, for example, distinguish whether the semi-conductive layer formed over the surface is ice, which might be dangerous, or surface water. The AC conductivity (in the above-mentioned range) and DC conductivity of water are substantially the same. With respect to ice, however, the AC conductivity and DC conductivity differ by two to three orders of magnitude. This difference in conductivity is measured by the respective ammeters and is compared in the current comparator. When the difference in conductivity is greater than a predetermined set point, the current comparator signals an icing alarm. At this point, for example, the voltage regulator subsystem can operate to apply a DC bias to the circuitxe2x80x94and thus to the interfacexe2x80x94at a desired field strength which sufficiently reduces the ice adhesion strength. According to one aspect of the invention, when ice is detected on an aircraft wing, the icing alarm initiates a feedback loop within the system which (a) measures ice conductivities, (b) determines appropriate bias voltages to reach minimum (or near minimum) ice adhesion conditions, and (c) applies a bias voltage to the ice-wing interface to facilitate ice removal.
Those skilled in the art should appreciate that the above-described system can be applied to many surfaces where it is desired to reduce ice adhesion strength, such as on car windshields, ship hulls and power lines. In such cases, if the surface material is weakly conductive, it is desirable to xe2x80x9cdopexe2x80x9d the surface material such that it is sufficiently conductive. Doping techniques are known to those in the art. Automobile tires, for example, can be doped with iodine to make the rubber conductive. Automobile glass, likewise, can be doped with either ITO or fluoride doped SnO2 to make the windshield an acceptable semiconductor.
However, in another aspect, the above described system and circuit are also applicable to situations where it is desirable to increase the ice adhesion strength. In this aspect, for example, when the icing alarm detects ice, the system activates the feedback loop to regulate applied DC voltages to the interface so as to increase ice adhesion. Situations and surfaces which can benefit from this system include, for example, the bottom soles of a person""s shoe (or shoes) and car tires on icy roads.
In still another aspect, the invention can include a variable ice adhesion/voltage control subsystem which increases and then decreases ice adhesion strength between ice and a surface, selectively. By way of example, cross country skis (or telemarking skis) ideally have higher friction when climbing an incline (or when descending an incline, in certain situations) and have lower friction when xe2x80x9cskiingxe2x80x9d down an incline. According to one aspect of the invention, the ice adhesion system and circuit described herein is attached in circuit with the skis and the operator can adjustably control ski friction selectively.
Other useful background to the invention may be found with reference to the following papers, each of which is incorporated herein by reference: Petrenko, The Effect of Static Fields on Ice Friction, J. Appl. Phys. 76(2), 1216-1219 (1994); Petrenko, Generation of Electric Fields by Ice and Snow Friction, J. Appl. Phys. 77(9), 4518-4521 (1995); Khusnatdinov et al., Electrical Properties of the Ice/Solid Interface, J. Phys. Chem. B, 101, 6212-6214 (1997); Petrenko, Study of the Surface of Ice, Ice/Solid and Ice/Liquid Interfaces with Scanning Force Microscopy, J. Phys. Chem. B, 101, 6276-6281 (1997); Petrenko et al., Surface States of Charge Carriers and Electrical Properties of the Surface Layer of Ice, J. Phys. Chem. B, 101, 6285-6289 (1997); and Ryzlikin et al., Physical Mechanisms Responsible for Ice Adhesion, J. Phys. Chem. B, 101, 6267-6270 (1997).
The invention is next described further in connection with preferred embodiments, and it will become apparent that various additions, subtractions, and modifications can be made by those skilled in the art without departing from the scope of the invention.